Ezio Melotti <ezio.melo...@gmail.com> added the comment:
IIUC this happens because StreamReader calls codecs.utf_8_decode without
passing final=1 [0], so when the decoder finds the trailing F4 it doesn't
decode it yet because it waits from the other 3 bytes (F4 is the start byte of
a 4-bytes UTF-8 sequence):
>>> b = b'A\xf5BC\xf4'
>>> chars, decnum = codecs.utf_8_decode(b, 'replace', 0) # final=0
>>> chars, decnum
('A�BC', 4) # F4 not decoded yet
>>> b = b[decnum:]
>>> b
b'\xf4' # F4 still here
>>> chars, decnum = codecs.utf_8_decode(b, 'replace', 0)
>>> chars, decnum
('', 0) # additional calls keep waiting for the other 3 bytes
>>> chars, decnum = codecs.utf_8_decode(b, 'replace', 1) # final=1
>>> chars, decnum
('�', 1) # when final=1 is passed F4 is decoded, but it never happens
While passing 1 makes the attached script work as expected, it breaks several
other test in test_codecs (apparently not all the decoders accept the 'final'
argument).
Also passing 1 should be done only for the last call: read can be called
several times with a specific size, and it shouldn't use final=1 until the last
call to avoid errors mid-stream.
[0]: see Lib/codecs.py:482
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Python tracker <rep...@bugs.python.org>
<http://bugs.python.org/issue12508>
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